The present invention relates to a sheet cassette in which sheets of paper can be stacked over another and which is removably mounted in a cassette containing portion of a main body of an apparatus that uses the sheets, and the invention relates to an information processing apparatus having a cassette containing portion in and from which the sheet cassette can be mounted and removed.
An information processing apparatus such as a printer, copying machine, or facsimile generally includes a sheet conveying portion for conveying a sheet of paper to an information processing portion to allow information processing such as a printing process to be performed on the sheet. Conventional information processing apparatuses generally have a configuration in which a sheet cassette for containing sheets of paper to be conveyed by such a sheet conveying portion is removably mounted in a main body of the apparatus.
Various types of sheet feeding mechanisms for such information processing apparatus have been proposed, the mechanisms having a sheet cassette which is removably mounted in a cassette containing portion of an apparatus main body with sheets of papers stacked therein one over another and which has a guide wall for regulating a sheet feeding direction provided at the end thereof in the mounting direction, a sheet feeding unit which is provided on the apparatus main body such that it can contact a surface of the uppermost sheet in the cassette containing portion and which has a pickup roller for exerting a feeding force acting toward the guide wall to the surface of the sheet in contact therewith, and a control unit for controlling the operation of the sheet feeding unit. The mechanisms are configured such that the sheets contained in the sheet cassette are supplied to a conveying mechanism provided ahead of the guide wall one at a time starting with the uppermost sheet. Another proposed configuration of a sheet cassette includes a base plate for lifting up sheets of paper stacked in the sheet cassette while urging them toward a pickup roller.
A maximum amount of sheets to be stacked in the sheet cassette is set in advance. When the cassette is in an overloaded state in which the number of sheets of paper exceed the specified amount, an excessive load is likely to be generated at a sheet feeding unit for feeding the sheets one at a time from the uppermost sheet. When an excessive load is generated at the sheet feeding unit, a part for driving the sheet feeding unit can be damaged, and there will be an increase in the contact pressure on the sheets. Thus, the sheets are more liable to be wrinkled or torn, which can result in errors in sheet feeding.
Sheet cassette configurations have been proposed for use in sheet cassettes having a base plate as described above in order to prevent the generation of an excessive load to a sheet feeding unit by reducing the urging force of a base plate even when sheets of paper are set in the excess of the specified amount (for example, see Japanese Patent Publication No. 2000-335755A).
The technique disclosed in Japanese Patent Publication No. 2000-335755A can be adopted only in sheet cassettes having a base plate for lifting up sheets of paper and cannot accommodate sheet cassettes of other types.
Although most sheet cassettes have a mark indicating such a specified amount in general, they can be easily loaded with sheets of paper in the excess of the specified amount and can be mounted in an apparatus main body even in such an overloaded state. When a sheet cassette contains sheets of paper in the excess of a specified amount, an excessive load is generated at a sheet feeding unit as described above, and problems can occur including damage to a part for driving the sheet feeding unit and wrinkling or tearing of paper.
Some sheet cassettes are provided with a cover to close a part of a top side of the same that is open in order to regulate a sheet containing space above the bottom of the cassette and prevent sheets of paper from being loaded in the excess of a specified amount. However, even such a cassette may be forcibly loaded with sheets of paper in the excess of the specified amount. In such a case, since the pile of paper is sandwiched between the cover and the bottom of the cassette, an error or jam can sometimes be caused by a heavy load generated when a sheet is fed even though a feeding force is exerted on the sheet by the sheet feeding unit during sheet feeding.
While a sheet cassette is liable to have problems when overloaded as described above, it has not been possible to prevent sheets of paper from being set in a sheet cassette in the excess of a specified amount.